Ecosystem Structures & Functions
Module 5: Food Chains and Food Webs
behaviour (e.g., prey get scared when predators are around and hide or move away). When the impact
of a predator on its prey’s ecology trickles down one more feeding level and affect the density and/or
grasshoppers and gazing animals contains growth hormones that stimulate root growth and the ability
of plants to regenerate new leaves, providing a mechanism for this positive feedback effect (Dyer et al
1993, 1995). So food webs exhibit partnerships and relationships between producers and consumers
and between different levels of consumers.
FOOD WEBS DESCRIBE DIRECT AND INDIRECT SPECIES INTERACTIONS IN A COMMUNITY: -
All species in the food webs are related to each other at different trophic levels. Different organisms
may occupy same trophic level. All species in the food webs can be distinguished into basal species
(autotrophs, such as plants), intermediate species (herbivores and intermediate level carnivores, such
as grasshopper and scorpion) or top predators (high level carnivores such as fox) (Fig. 5.4). Therefore,
grouping all of them into different functional groups or tropic levels helps us simplify and understand
the relationships among these species.
FOOD WEBS CAN ALSO BE USED TO ILLUSTRATE INDIRECT INTERACTIONS AMONG SPECIES: -
Generally, two types of effects occur when the interactions between two species are influenced by a
third species. If we manipulate one species in a food web, it may change the population of species that
neither eat nor are eaten by the manipulated species. These effects include trophic cascades, where
predators enhance producer growth by feeding on consumer species, and keystone predation, where
predators consume dominant competitors, thus allowing inferior competitors to persist.
Trophic Cascades: In a prey-predator relationship, predators can impact both prey abundance and
behaviour of the prey's prey, the impact is known as a feeding or trophic cascade (Fig. 5.7). In this
condition, by controlling densities and/or behaviour of their prey, predators indirectly benefit and
increase the abundance of their prey's prey.
Trophic cascades by definition must occur across a minimum of three feeding levels, although
evidence of 4- and 5-level trophic cascades have been shown in nature, but are far less common. In
figure 5.7, the bird/caterpillar/plant relationship is an example of a trophic cascade. The plant is